Metal-oxide semiconductor field effect transistor (MOSFET) such as lateral MOSFETs are often used in high voltage (i.e., greater than 200 volts) applications such as switching device telecommunication systems or as off-line switching regulators in AC to DC voltage converters. In a typical high voltage lateral MOSFET, the source and drain regions are separated by an intermediate or drift region. The gate structure is disposed adjacent to the channel region of the device. In the on state, a voltage is applied to the gate to form a conduction channel region between the source and drain regions, which allows current to flow through the device. In the off state, voltage applied to the gate is sufficiently low so that a conduction channel does not form, and thus current flow does not occur. During the off state, the device must support a high voltage between the source and drain regions.
ON resistance (RON) is an important performance figure of merit for MOSFET switching devices. ON resistance is the ohmic resistance that exists between an input and an output pin of a MOSFET switch when the switch is closed and passing a signal. ON resistance correlates to how much signal attenuation will result as the signal passes through the device. Another important figure of merit is specific on resistance (RSP), which is the product of RON and surface area, or RON*Area. A lower RON*Area allows a designer to use a smaller high voltage lateral MOSFET to meet ON resistance requirements for a given application, which reduces the area and cost of a power integrated circuit.
One problem with conventional high voltage lateral MOSFETs is that techniques and structures that tend to maximize breakdown voltage (VBD) adversely affect RON and vice versa. For example, typical lateral MOSFETs require a lower doping concentration in order to support a higher VBD, which increases specific on-resistance (RSP).
To overcome this problem, several designs have been proposed in an attempt to provide acceptable combinations of high breakdown voltage and low RON*Area. For example, devices have been designed with one or more reduced surface field (RESURF) regions and/or regions of localized doping (also referred to as super junction or multiple conduction structures). However, these designs require expensive wafer processing involving multiple masking and ion implant steps, very deep diffused body regions or contacts (e.g., 30 to 40 microns deep), and/or expensive silicon on insulator substrates, which increase the cost of chip manufacturing
Accordingly, a need exists for cost effective structures and methods that improve the Ron*Area performance of lateral MOSFET devices while maintaining high blocking voltage capability and manufacturing flexibility.